Conventionally, various improvements have been made to lubricating base oils and antioxidants for use in lubricating oils for air compressors. For example, Patent Document 1 discloses a lubricating oil composition for an air compressor obtained by blending a lubricating base oil having a viscosity index of 120 or higher with an amine-based antioxidant, such as alkylphenyl α-naphthylamine or p,p′-dialkyldiphenylamine.
Because lubricating oils for air compressors are used under severe conditions for a long period of time, it is required to prevent oxidation of the lubricating oils under high temperature for a long period of time. However, the amount of the antioxidant blended into the lubricating oil composition of Patent Document 1 is too small to suppress oxidation to a satisfactory extent under high temperature. One possible method for preventing oxidation is to increase the amount of antioxidant. However, because a mineral oil-based base oil having a low solubility to the antioxidant is used as the lubricating base oil of the lubricating oil composition in Patent Document 1, the increase of the amount of antioxidant causes the generation of sludge, resulting in failures such as compressor trouble.
On the other hand, polyglycol-based or ester-based synthetic base oils have the advantages of being less likely to generate sludge, due to high solubility of various additive thereto. Thus, various additives could be blended at high ratios into a lubricating oil using a synthetic base oil. However, if an antioxidant conventionally used in synthetic base oils is blended into a synthetic base oil at a high ratio without modification, a problem, such as an increase in viscosity to a level that makes it unusable as a lubricating oil or an unintended increase in acid value, may occur. The current situation is that proper selection of antioxidant suitable for synthetic base oils has yet to be made.
As described above, a lubricating oil for an air compressor has not been conventionally developed which can successfully stabilize the acid value under high temperature over a long period of time and can prevent the generation of sludge.